The present invention relates to an anchoring arrangement in general, and more particularly to a rock anchoring arrangement for use in tunnel, mine, and similar underground constructions, especially such which are subjected to considerable convergences.
Rock anchoring arrangements which have hitherto been used in tunnel and mine constructions have a limited extensibility of no more than approximately 15%. This means, generally speaking, that they can only be employed in situations where only relatively slight movements of the rock strata can be expected. As a result of the advancement into steadily increasing depths, and as a result of the attendant proportionately increasing convergences, there exists, in practice, a need for the provision of rock anchors which would render it possible to keep the rock under control, even when the movement of the strata is substantial.
One conventional rock anchor of this general type is disclosed on page 5 of the issue No. 105 of Kurznachrichten, published by Steinkohlenbergbauverein in December 1979. This rock anchor for substantial convergences comprises an anchoring rod which is adhesively secured within a steel pipe that is filled with a synthetic resin mortar. At the inner or leading end of the anchoring rod, as considered in the direction of introduction of the rock anchor into the associated bore in the rock formation, there are provided radially extending protuberances which are being pulled through the adhesive which secures the anchoring rod in the anchoring sleeve or pipe as the anchoring rod is loaded in its longitudinal direction. Thus, the adhesive, which consists of a synthetic resin material, forms a severable lining. The force required for pulling the protuberances through the severable lining determines the resistance of the rock anchoring arrangement to extension. The inner or leading end of the anchoring rod, which carries the protuberances, has a diameter which is reduced relative to the remainder of the anchoring rod, to present a possibility for the particles or chips of the material detached from the severable lining by the protuberances to escape from the space between the anchoring rod and the anchoring sleeve, so that they do not become packed ahead of the protuberances and either increase the resistance of the anchoring arrangement to further extension of elongation, or widen the sleeve, or both.
In view of the necessary strength of the protuberances and their being embedded in the anchoring rod, however, only a very limited escape gap can be provided between the anchoring rod and the anchoring sleeve. As a result of this, the particles, slivers or chips of the severed material which are detached from the anchoring sleeve by the action of the protuberances on the severable lining must pass through this relatively narrow gap. This results in a blocking action at the severable lining, which becomes progressively worse with increasing speed of displacement. Eventually, this may result in a situation where the force needed for extending the anchoring arrangement substantially exceeds that originally contemplated, which may result in the destruction of the anchoring arrangement.
In addition to the afore-mentioned disadvantage of non-uniform extension force, and the disadvantage of high consumption of material, which is attributable to the substantial axial length of the anchoring sleeve and the considerable axial length of the anchoring rod which carries the severing protrusions, there are present, in this conventional anchoring arrangement, further disadvantages which considerably limit the usability of the conventional rock anchoring arrangement in practical applications.
If the anchoring sleeve were to be adhesively secured, for instance, at the bottom region of the bore provided therefor in the rock formation, it would be necessary or even mandatory to give the bore a substantial diameter. There also exists the possibility that, in the event of rock shift transversely of the elongation of the anchoring arrangement, the anchoring rod would be pulled out from the anchoring sleeve, whereby the effective extension path of the rock anchor arrangement in its longitudinal direction is diminished. Moreover, it would only make sense to fix the rock anchor in the bore via its anchoring sleeve. If the anchoring rod were also adhesively attached, the lengthening effect of the anchoring arrangement, which is desired in order for the rock anchoring arrangement to be able to adapt itself to considerable convergences, would be defeated. In this case, there would then be obtained only a rock anchoring arrangement conforming to the initially disclosed rock anchoring arrangement which is used in constructions involving only slight movements of the strata.
On the other hand, should the anchoring sleeve be adhesively attached at the region of the open end of the bore, the anchoring sleeve would have to be equipped with an additional anchoring extension reaching to the bottom of the bore. Though such a construction would avoid the disadvantage of having to maintain a relatively large bore diameter all the way to the bottom of the bore because it could then be drilled in steps, in this instance the distance between the open end of the bore and the anchoring sleeve would increase upon loading of the rock anchoring arrangement, because the anchoring sleeve would travel into the rock formation. In addition thereto, rock or adhesive mortar would be displaced by the anchoring sleeve during the process. The necessary force transmitted as a result thereof from the anchoring sleeve to the anchoring rod depends on the solidity of the rock and/or the strength of the adhesive mortar. Since this force cannot be determined accurately and, accordingly, can grow higher than the desired extension force, there exists the risk that the adhesively attached anchoring rod could break before the desired extension force was even reached. In this manner, the very reason for providing the rock anchoring arrangement, that is, to hold the rock in place, would be defeated.